D.C. GENERATORS PRINCIPLE OFOPERATIONDC generator converts mechanical energy intoelectrical energy. when a conductor move in a magneticfield in such a way conductors cuts across a magneticflux of lines and e.m.f. produces in a generator and it isdefined by faradays law of electromagnetic inductione.m.f. causes current to flow if the conductor circuit isclosed.
Faradays lawsFirst Law :Whenever the magnetic flux linked with a circuit changes, an e.m.f. isalways induced in it.orWhenever a conductor cuts magnetic flux, an e.m.f. is induced inthat conductor.Second Law :The magnitude of the induced e.m.f. is equal to the rate of changeof flux linkages.
Faradays Law of Electromagnetic InductionA changing magnetic flux through a loop or loops of wire induces an electromotiveforce (voltage) in each loop.
Lenz’s Law“The induced currents in a conductor are in such a direction as tooppose the change in magnetic field that produces them..”“The direction of induced E.M.F in a coil (conductor) is suchthat it opposes the cause of producing it..”
Flemings Right Hand Rule• The Thumb represents the direction of Motion ofthe conductor.• The First finger (four finger) represents Field.• The Second finger (Middle finger) representsCurrent
GeneratorsBasic operation of the generatorAs the loop rotates, the magnetic flux throughit changes with timeThis induces an e.m.f and a current in theexternal circuitThe ends of the loop are connected to sliprings that rotate with the loopConnections to the external circuit are made bystationary brushes in contact with the slip rings
Working Principle of D.C GeneratorSchematic diagram of a simple DCGenerator1st half cycle(00 to 1800 ) Path of currentABR1B1MLR2B2CD2st half cycle(1800 to 3600) Path of currentDCR2B1MLB2R1BA
1)Yoke- Acts as frame of the machine- Mechanical support- low reluctance for magnetic flux- High Permeability-- For Small machines -- Cast iron—low cost-- For Large Machines -- Cast Steel (Rolledsteel)Large DC machineSmall DC machine
2)pole cores and pole shoesa) Pole core (Pole body) :- --Carry the field coils--Rectangle Cross sections-- Laminated to reduce heat losses--Fitted to yoke through boltsb) Pole shoe:- Acts as support to field polesand spreads out fluxPole core & Pole shoe are laminated of annealed steel(Of thickness of 1mm to 0.25 mm)
4)commutator:--Hard drawn copper bars segments insulated from eachother by mica segments (insulation)-- Between armature & External circuit-- Split-Rings (acts like Rectifier AC to DC )
5&6 Bearings and Brushes5)Brushes and brush gear:-Carbon, Carbon graphite, copper used to Collects currentfrom commutation (in case of Generator)6)Shaft and bearings:-Shaft-- Mechanical link between prime over and armatureBearings– For free rotation
Lap Winding: are used in machines designed for low voltage and high currentarmatures are constructed with large wire because of high currentEg: - are used is in the starter motor of almost all automobilesThe windings of a lap wound armature are connected in parallel. Thispermits the current capacity of each winding to be added and provides ahigher operating current.No of parallel path, A=P ; P = no. of poles
Wave winding:are used in machines designed for high voltage and low currenttheir windings connected in seriesWhen the windings are connected in series, the voltage of each windingadds, but the current capacity remains the sameare used is in the small generator.No of parallel path, A=2,
Commutation process in D.C GeneratorCommutation is the positioning of the DC generator brushes so that thecommutator segments change brushes at the same time the armature currentchanges direction.
The total losses in a dc machine1.Cu losses2.Iron losses3.Mechanical lossesCupper losses are mainly due to the current passing through the winding.1.Armature cu losses (30 to 40% of full load losses)Cu losses 2.Shunt field cu losses(20 to30% of full load losses)3.Series field cu losses
1.Cu lossesArmature cu losse s= Ia2 RaRa=Armature resistance , Ia= Armature current--Losses due to brush contact resistance is usually include in armature cu lossesShunt field cu losses = Ish2RshRsh=Shunt field resistance, Ish=Shunt field currentSeries field cu losses = Ise2RseRse=Series field resistance , Ise=Series field current
2.Ironlosses (Magnetic losses) (20 to 30% of full load losses)1)Hysteresis losses2)Eddy current losses
1)Hysteresis losses (Wh)The losses is due to the reversal of magnetisation of the armature coreEvery portion of the rating core passes under N and S poles alternately. There by attaining S and Npolarity respectively. The core undergoes one complete cycle of magnetic reversal after passingunder one pair of poles.P=No. of polesN= Armature speed in rpmfrequency of magnetic reversalsf=NP120The losses depends upon the volume and B max and frequency of reversals.Hysteresis losses is given by steinmetz formulaWh=η B1.6maxf V watsV=Volume of the core in m3η= Steinmetz hysteresis coefficient
2)Eddy current losses:-(We)when the armature core rotates, it cuts the magenetic flux hence an e.m.finduced in in the body of the core according to faradays law of electro magneticinduction. This e. m.f through small sets up large current in the body of the coredue to its mall resistance. This current is known as “Eddy Current”We=k B2maxf2t2v2 wattsBmax=maximum flux densitiesf=Freequency of the magenetic reversalsv=volume of the armaturecoret=Thick ness of lamination
Efficiency of D.C GeneratorEfficiency of generator is defined as the ratio of output power to input powerEfficiency (η) =output 100inputinput=output+ losses (or) output=input-lossesFor D.C generator input mechanical & output electrical
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